You might want to hop into the herbalism forum to read about the ongoing concerns about the internal use of comfrey.

How long do you want your chickens and sheep to live? Comfrey is a good source of many nutrients, however.

Some strains of comfrey may contain less of the harmful substance under discussion over there, but I don't know which one that is.

When I had ducks, I found they greatly enjoyed radish greens, as well as any other greens left over from pulled up crops, second- quality tomatoes, Japanese beetles, etc. (preferably thrown in their water tub). Ducks are the pigs of the poultry world, however; possibly chickens would be more particular.